Implantable cardiac defibrillators (ICD's) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in a pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. An arrhythmia detector detects accelerated arrhythmias, such as tachycardia or fibrillation. When such a tachyarrhythmia is detected, a pulse generator delivers electrical therapy to the patient's heart. A therapy for tachycardia may be anti-tachycardia pacing and a therapy for fibrillation may be a defibrillating shock. Such therapies are well known.
With ventricular tachycardia (VT) the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a ventricular tachycardia is typically not immediately fatal. However, ventricular fibrillation (VF) is an immediately life threatening tachyarrhythmia, wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs.
With atrial tachycardia (AT), the atria of the heart beat rapidly at an abnormally high rate. This can cause the ventricular to in turn beat at a high rate. Cardiac output is downed. The patient may experience dizziness or feel fatigued. Although not immediately life threatening, it can also be unpleasant to a patient.
Atrial fibrillation is a common atrial tachyarrhythmia and can occur suddenly. It results in rapid and chaotic activity of the atrial of the heart. The chaotic atrial activity in turn causes the ventricular activity to become rapid and variable. Although not life threatening, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition to strokes, symptoms of atrial fibrillation may include fatigue, syncope, congestive heart failure, weakness and dizziness.
From the foregoing, it may be readily understood that it is most desirable to treat tachyarrhythmias as quickly as possible to return the heart to normal sinus rhythm (NSR). Tachycardia may be treated quite effectively with ATP while fibrillation generally requires a defibrillating shock. However, defibrillating shocks can cause discomfort and trauma to a patient while ATP is generally not perceptible. Hence, to achieve the most effective and the quickest return of tachyarrhythmias to NSR with the least amount of discomfort to the patient, it would be desirable to match the therapy to the tachyarrhythmia.
The need for therapy matching techniques becomes even more apparent in view of some recent research which has shown that the majority of very fast rhythms, normally treated as VF with a defibrillating shock, are actually monomorphic VT. These arrhythmias have traditionally been treated as a VF in ICDs because there was no reliable means of separating VFs, which require shocking, from VTs, which may be treated with ATP. Such research showed that approximately 50% of these high-rate tachyarrhythmias were, in fact, monomorphic tachycardias, and not VF.
Hence, there is a need in the art for an implantable cardiac stimulation device capable of distinguishing tachycardias from fibrillation and providing an appropriate therapy. There is also a need to accomplish the foregoing which assures that the patient is receiving the most appropriate therapy in the shortest possible time.